I’m guessing the answer is...one!
Haiti: Violent revolution to get rid of slavery.
Brazil: Emperor overthrown because of slavery.
Most new world lands with slavery were either colonies that had to go along with what the mother country decided, or they had so few slaves that slavery wasn't much of an issue.
But in fact, in many countries, emancipation happened during or after the bloody wars for independence from Spain.
However you look at it the answer was more than one.
Perhaps I should have written, “How many democracies used a Civil War to end slavery?”
After all, a revolt by the slaves to gain freedom really wasn’t an option in the USA.
Brazil:
“Brazil’s 1877–78 Grande Seca (Great Drought) in the cotton-growing northeast led to major turmoil, starvation, poverty and internal migration. As wealthy plantation holders rushed to sell their slaves in the south, popular resistance and resentment grew, inspiring numerous emancipation societies. They succeeded in banning slavery altogether in the province of Ceará by 1884...
...In 1872, the population of Brazil was 10 million, and 15% were slaves. As a result of widespread manumission (easier in Brazil than in North America), by this time approximately three quarters of blacks and mulattoes in Brazil were free.
Slavery was not legally ended nationwide until 1888 by the Lei Áurea (”Golden Act”), a legal act promulgated on May 13 by Isabel, Princess Imperial of Brazil. In fact, it was an institution in decline by this time (since the 1880s the country began to attract European immigrant labor instead). Brazil was the last nation in the Western world to abolish slavery, and by abolition had imported an estimated total of four million slaves from Africa. This was 40% of all slaves shipped to the Americas.”
https://en.wikipedia.org/wiki/Slavery_in_Brazil#Steps_towards_freedom
More on Brazil, since I knew nothing about it:
” Although initially a conservative ruler, Pedro II eventually recognized the inherent unfairness of the slavery system affecting so many millions of his subjects.
Pedro gradually passed laws that liberated his subjects. By the late 1880s, it was just a matter of time before the emperor abolished slavery in Brazil altogether. Unfortunately for the Crown, Brazilian landowners and the country’s military leadership were not keen on the liberalizing policies of Pedro II. The abolition of slavery subjected landowners to higher capital investment in manpower, and since these conservative groups were the mainstay of the military, the armed forces were predisposed to side with the land-owning classes. Dom Pedro was traveling in Europe when Princess Isabel (shown 3rd from left, standing), acting as regent in her father’s stead, passed a law abolishing slavery in Brazil on May 13, 1888.
This law, commonly known as the Golden Law, not only brought international praise to the Brazilian imperial family, but also condemned the Crown. The landowners quickly organized and built opposition to the monarchy. Revolts broke out in different regions of the country. In many instances Brazil’s republican neighbors, countries that had always resisted having an emperor in Latin Americ, helped these revolts. Princess Imperial Isabel’s decree eventually led to the proclamation of the Brazilian Republic on November 16, 1889.”
http://www.aaregistry.org/historic_events/view/brazil-abolishes-slavery
The Emperor was overthrown in part FOR ending slavery.